A gas carburizing method is available as one of the methods, i.e., carburizing methods, used for hardening only the surface layer of a steel product by diffusing carbon in the surface layer of the steel product. A conventional apparatus for practicing the gas carburizing method has a main furnace and a conversion furnace. An atmospheric gas having a carburizing property is sent to the main furnace until it is filled. A steel product, i.e., a workpiece is heated in the main furnace and is thus carburized. The conversion furnace generates a carrier gas, e.g., an endothermic conversion gas (to be referred to as an RX gas hereinafter). A carburizing gas, e.g., a hydrocarbon gas (a propane or city gas), is added to the generated carrier gas to increase its carburizing property, and the resultant gas mixture is supplied to the main furnace. The carbon potential of the atmosphere gas to be supplied to the main furnace is adjusted by adjusting the adding amount of the hydrocarbon gas.
However, the conventional method and apparatus described above require a conversion furnace in addition to a main furnace. Therefore, a heat energy is needed for the conversion furnace resulting in increase in the cost. Since a heater, a retort and the like used in the conversion furnace are expendable supplies, their maintenance is costly. Since the conversion furnace requires an expensive catalyst to effectively generate the RX gas, the cost is further increased.
Since the RX gas contains a component which is unstable at high temperatures, it is rapidly cooled in the exit of the conversion furnace so that the composition of the RX gas may not be changed. Therefore, a cooled RX gas is supplied to the main furnace and energy loss in the main furnace becomes undesirably large.
In the conventional method, the carbon potential in the atmospheric gas in the main furnace is adjusted by adjusting the amount of the hydrocarbon gas added to the RX gas. However, even a little change in amount of the hydrocarbon gas to be added results in a great change in carbon potential of the atmospheric gas. Therefore, adjustment of the carbon potential of the main furnace tends to be inaccurate. Particularly, when the state of the atmospheric gas changes in a comparatively short period of time, as in a continuous gas carburizing furnace, adjustment of the carbon potential of the atmospheric gas tends to be more inaccurate. As a result, soot attaches to the surface of the workpiece which interferes with carburization and carburizing is interfered.
Load and unload chambers are provided before and after the conventional main furnace, respectively. Entrance and exit doors are provided to the load and unload chambers, respectively. Intermediate doors are provided between the load chamber and the main furnace and between the main furnace and the unload chamber. When a workpiece is to be loaded, the entrance and intermediate doors of the load chamber are alternately opened/closed. When a workpiece is to be unloaded, the intermediate and exit doors of the unload chamber are alternately opened/closed. As a result, the atmospheric gas in the main furnace is prevented from flowing to the outside. Generally, the gas temperature of the main furnace is maintained at about 900.degree. C., and the gas temperature of the load and unload chambers is maintained at about 500.degree. C.
When the entrance, exit, or intermediate door is opened/closed, the gas pressure in the load or unload chamber sometimes becomes negative since the gas flows out of the furnace. In this case, when external air should flow into the load or unload chamber, the high-temperature gas in the load or unload chamber is mixed with the air and may cause an explosion. In order to prevent this, conventionally, an RX gas is supplied to the load and unload chambers from the conversion furnace so that the pressure in them may not be negative. This results in a large RX gas consumption, which is an economical disadvantage.
In another conventional apparatus, an inert gas such as nitrogen is supplied to the load and unload chambers when the pressure in them becomes negative. However, this apparatus requires a separate inert gas source, e.g., a cylinder or the like containing nitrogen. As a result, the entire apparatus becomes complex and is disadvantageous in terms of economy since a large amount of inert gas is consumed. When the inert gas is supplied, the composition of the atmosphere gas in the main furnace is changed, and sometimes the quality of the workpiece is adversely influenced.